CYLINDER FOR A RECIPROCATING HERMETIC COMPRESSOR

Abstract: Cylinder for a reciprocating hermetic compressor, of the type comprising: a cylinder block presenting an internal cylindrical cavity (2), in which inside reciprocates a piston (3) between a lower dead point and an upper dead point, said reciprocating piston (3) presenting an upper end face (3a), the radial distance between the lateral wall of the reciprocating piston (3) and the adjacent lateral wall of the cylinder (2) varying throughout the cylinder length, in order that, at the region of the cylinder (2) adjacent to the portion thereof where there is reached the upper dead point condition, said radial distance is minimum.

Description: CYLINDER FOR A RECIPROCATING HERMETIC COMPRESSOR Field of the Invention The present invention refers to a cylinder for a reciprocating hermetic compressor of the type used in small refrigerating systems. Background of the Invention In the reciprocating hermetic compressors, the reciprocating motion of a piston inside a cylindrical cavity, such as a cylinder provided in the cylinder block of said hermetic compressors, define suction and compression strokes of refrigerant gas during each operative cycle of the piston. In these operative cycles, the refrigerant gas is aspirated from a suction chamber towards the cylinder inside, through a suction orifice provided on a valve plate, during the suction stroke and it is discharged during the compression stroke from the inside of the cylinder to a discharge chamber, through a discharge orifice provided on said valve plate. The suction and compression strokes of the operative cycle of the reciprocating piston are defined by the stroke end positions of said piston, respectively known as upper dead point position and lower dead point position. In these compressors, the cylinder, within which reciprocates the piston, presents a constant cylindricity throughout its length, said cylindricity defining, in a transversal profile, walls that are parallel to the respective walls of a reciprocating piston mounted inside said cylinder. The radial distance between the cylinder wall and the piston wall is calculated in order to be sufficient to receive, by superficial adherence, a lubricant oil film that reduces the friction between the walls of the cylinder and reciprocating piston, mainly when said piston comes closer to the upper dead point, where the pressure rises quickly with the advance of the reciprocating piston. In these constructions, with the pressure rise inside the cylinder during compression, there is an increase in the leakage of gas that is being compressed to the inside of the compressor shell and, consequently, to the low pressure portion of the refrigerating system, through a radial clearance existing between the adjacent walls of the cylinder and reciprocating piston. A reduction in the radial clearance between the cylinder and piston walls, from the lower dead point up to the upper dead point, though reducing the problem of gas leakage during compression, contributes to reduce the mechanical efficiency of the compressor, due to power losses by friction caused by the motion of said piston in relation to the oil film. Disclosure of the Invention Thus, it is an object of the present invention to provide a cylinder for a reciprocating hermetic compressor, whose construction allows to obtain, in each position of the piston throughout the length of the cylinder during the operative cycle of the reciprocating piston, minimum mechanical losses for the compressor and minimum gas leakage at the low pressure portion of the refrigerating system to which the hermetic compressor is associated. Another object of the present invention is to provide a cylinder such as cited above, which maintains unaltered the performance of said cylinder and piston during the whole useful life of the compressor. These objectives are attained through a cylinder for a reciprocating hermetic compressor of the type comprising: a cylinder block presenting an internal cylindrical cavity, in which inside reciprocates a piston between a lower dead point and an upper dead point, said reciprocating piston presenting an upper end face, the radial distance between the lateral wall of the reciprocating piston and the adjacent lateral wall of the cylinder, according to a plane containing the upper end face of the reciprocating piston, varying at least along the cylinder length, in order that, at the region of the cylinder adjacent to the portion where there is reached the upper dead point condition, said radial distance is minimum. Brief Description of the Drawings The invention will be described below, with reference to the attached drawings, in which: Fig. 1 illustrates, in a longitudinal section view, a reciprocating piston disposed inside a cylinder, which is closed by a valve plate and cylinder head, according to the state of the art; Fig. 2 illustrates an enlarged view of the assembly of figure 1, without the connecting rod of the piston, the latter being at its upper dead point; and Figures 3a and 3b illustrate, in an enlarged longitudinal section view, similar views to that of figure 2, but presenting a cylindrical cavity, which is shaped according to two different embodiments of the present invention. Best Mode of Carrying Out the Invention Reciprocating hermetic compressors comprise a cylinder block 1, which is lodged inside a hermetic shell (not illustrated) and which presents a cylindrical cavity such as a cylinder 2, in which inside reciprocates a piston 3. The cylinder block 1 presents a pair of opposite faces, to which are opened the ends of the cylinder 2. Against one of said opposite faces of the cylinder block 1 there are attached, through non- illustrated gaskets, a valve plate 5 and a cylinder head 6, which forms together with said valve plate 5 two internal cavities, defining a suction chamber 8 and a discharge chamber 9. The cylinder 2 maintains a selective fluid communication with said suction chamber 8 and discharge chamber 9, through respective gas suction orifice 5a and gas discharge orifice 5b, provided on the valve plate 5, eccentrically to the geometric axis of the cylinder 2, and onto which are mounted respective suction valve 10 and discharge valve 11, which operate synchronously during the suction and discharge strokes of the operative cycle of the piston 3. The reciprocating piston 3 presents a free end face 3a, facing the valve plate 5, and an opposite end 3b, by which said reciprocating piston 3 is driven through a connecting rod 20. According to the driving system presently used in hermetic compressors, the reciprocating piston 3 presents a continuous linear movement, having a suction stroke and a discharge stroke, which begin, respectively, at the upper dead point and at the lower dead point. In this construction, the cylinder 2 presents a cylindricity that remains constant throughout its length, from the edge thereof adjacent to the upper dead point region of the reciprocating piston 3 and close to the valve plate 5, up to another opposite edge, which is adjacent to the lower dead point region of said piston 3, defining, in a transversal profile, a parallelism between the adjacent walls of the cylinder 2 and piston 3. The diameter of said cylinder 2 is defined so that its lateral wall and the lateral wall of the piston 3 keep a constant minimum mutual radial distance, which is necessary to allow a fine oil film to be retained between the surfaces of the lateral walls of the cylinder and piston, in order to minimize the friction between said cylinder and piston during the motion of said piston inside the cylinder 2. This lubrication is mainly required during the final phase of the compression stroke, when the pressure inside the cylinder 2 increases rapidly. Nevertheless, when the piston is close to the upper dead point, this oil film is not enough to avoid the friction wear between the reciprocating piston 3 and the cylinder 2. In order to minimize this wear, it would be necessary to increase the distance between the cylinder 2 and the piston 3. Though the increase of said distance does not alter the results of gas suction during the corresponding cycle of the piston, said distance, during compression, implies an increase of gas leakage to the low pressure region of the refrigerating system. Said gas leakage is even more significant as the reciprocating piston approaches the upper dead point, thereby resulting in efficiency loss for the compressor. According to the present invention, as illustrated in figures 3a-3c, the cylinder 2 presents, between its opposite ends, a diametral variation calculated in order to optimize the gas pumping inside the cylinder, by action of the reciprocating piston 3, said optimized operation resulting in the minimization of the friction losses between said cylinder and piston, as well as losses due to leakage of the gas being compressed at the low pressure region of the refrigerating system. This diametral variation is obtained according to at least a plane containing the free end face 3a of the reciprocating piston 3. In a preferred form of the invention, the cylinder is defined as a conical cavity, whose walls maintain a constant conicality throughout the length of the cylinder, between the upper dead point and lower dead point regions. This conicality causes a gradual linear reduction of the radial distance between the adjacent walls of the cylinder 2 and piston 3, between said regions. In this solution, the ideal optimum point is located approximately at the central region of the cylinder 2. The ideal optimum point defines a region of the cylinder length, where the piston-cylinder radial distance is such that, during compression, the losses due to friction and to compressed gas leakage are optimum. Above this region, the cylinder 2 presents a radial distance from the reciprocating piston 3 that is smaller than the distance at said ideal optimum point, while, below said region, this distance is larger than said distance at the ideal optimum point. Nevertheless, larger distances between the cylinder 2 and piston 3 at the region of the latter adjacent to the lower dead point, will not result in gas leakage, since at this region the pressure difference between each point of said piston motion inside the cylinder 2 is minimum. In a constructive alternative, the cylinder 2 could present at this region a substantially reduced conicality, or again a cylindrical shape. The larger distance between the piston 3 and cylinder 2, when obtained close to the lower dead point, would serve to reduce the friction losses between said piston 3 and cylinder 2. Though presenting a simpler construction, the solution having a cylinder 2 with a constant conicality throughout its length presents an inconvenience. The minimum radial distance between the cylinder 2 and reciprocating piston 3 occurs only at the upper dead point region, between the peripheral edge of the free end face of the piston 3 and the wall of the cylinder 2, thereby allowing a certain escape of lubricant oil between the piston and cylinder walls, due to the pressure existing at this region, consequently increasing the gas leakage. In order to avoid such problem, the region of the cylinder immediately adjacent to the portion thereof where the upper dead point is defined, should present a constant radial distance, i.e., a constant cylindrical distance. At this cylindrical portion of reduced radial clearance, the oil forms a barrier against gas flow, thereby avoiding gas leakage even in high pressure conditions. In another solution, the variation of the cylinder- piston radial distance is obtained by varying the profile of said cylinder 2 throughout the length thereof. In this case, the diameter of the cylinder is calculated, for each position of the piston stroke, in function of some operative parameters of said piston, such as pressure in each said position and motion velocity between each two consecutive positions, i.e., the time that said piston stays in each of said positions.

Claims: CLAIMS 1. Cylinder for a reciprocating hermetic compressor, of the type comprising: a cylinder block presenting an internal cylindrical cavity (2) , in which inside reciprocates a piston (3) between a lower dead point and an upper dead point, said reciprocating piston (3) presenting an upper end face (3a) , characterized in that the radial distance between the lateral wall of the reciprocating piston (3) and the adjacent lateral wall of the cylinder (2) , according to a plane containing the upper end face (3a) of the reciprocating piston (3) , varies at least throughout the cylinder length, in order that, at the region of the cylinder (2) adjacent to the portion thereof where there is reached the upper dead point condition, said radial distance is minimum. 2. Cylinder, according to claim 1, characterized in that the variation of said radial distance throughout the length of the cylinder (2) is linear at least at a median portion of said length. 3. Cylinder, according to claim 2, characterized in that the variation of said radial distance throughout the length of the cylinder (2) is substantially null at a portion of said cylinder length adjacent to the upper dead point of said piston stroke. 4. Cylinder, according to claim 2, characterized in that the variation of said radial distance between the adjacent walls of the cylinder (2) and reciprocating piston (3) is linear throughout the length of said cylinder (2) . 5. Cylinder, according to claim 1, characterized in that, for each stroke position of the reciprocating piston (3) along the length of the cylinder (2) , the radial distance between the lateral walls of the cylinder (2) and reciprocating piston (3) is constant on a plane containing the free end face (3a) of said reciprocating piston (3) .

Inventors: LILIE DIETMAR ERICH BERNHARD, NUNES JUNIOR ERNANI PAUTASSO, LILIE, DIETMAR, ERICH, BERNHARD, NUNES JUNIOR, ERNANI, PAUTASSO
Applicants: BRASIL COMPRESSORES SA, LILIE DIETMAR ERICH BERNHARD, NUNES JUNIOR ERNANI PAUTASSO, EMPRESA BRASILEIRA DE COMPRESSORES S/A. - EMBRACO, LILIE, DIETMAR, ERICH, BERNHARD, NUNES JUNIOR, ERNANI, PAUTASSO
Application year: 1994
Main classification: F04B39
IPC codes: F04B39